In a common type of apparatus for splicing or welding optical fibers made from basically silica or quartz to each other a high voltage applied to a pair of electrodes is used to generate an electric arc enclosing end regions of the fibers to be spliced to each other. In the electric arc the temperature is high causing material of the optical fibers inside the arc to melt and by slightly pressing the ends of the fibers to each other (this operation called “overlap”) a fusion-splice is obtained. The temperature in the fusion region must be above 1800° C. or generally about 2000° C. in order to melt the silica material of the fibers. When the fibers are heated to such high temperatures, some material from the fibers is evaporated and deposited on surfaces of the electrodes, in particular on the points or tips of the electrodes. The deposited material mainly appears as silica particles attached to the electrode surfaces, see the photograph in FIG. 1. A phenomenon which is directly correlated with the deposition of such particles is called “arc-walk” and comprises that the position of the center of the electric arc varies from one splice to the next one. This spatial movement of the arc is mainly caused by the dynamic process of particles being inhomogenously or non-uniformly deposited on the surfaces of the electrodes. The thickness of the deposited particle layer on the electrodes keeps on changing from place to place since part of the deposited particles are removed from the surfaces when the arc is active and new particles are deposited when the arc is turned off. Thus it is found, that instead of being ignited at the tips of the electrodes, the arc is often started from different spots at some generally small distance of the tips, this being dependent on the coverage of the deposited particle layer. As a consequence, arc-walk occurs. Arc-walk could result in a significant variation of the temperature distribution inside the fusion region which can in turn result in bad splices having high optical losses.
A few methods have been developed which involve temperature control and which thus can to some extent handle arc-walk, see e.g. U.S. Pat. No. 5,772,327 for Wenxin Zheng, “Automatic fusion-temperature control for optical fiber splicers”, and U.S. Pat. No. 5,909,527 for Wenxin Zheng, “Automatic current selection for single fiber splicing”. The methods disclosed in these patents are based on compensation of the electric current flowing between the electrodes in the fusioning operation in order to maintain a desired temperature inside the arc and in the fusioning region. However, only temperature control is not sufficient to handle the situation of large distances of arc-walk, such as distances e.g. larger than about 30 μm. This is because a high current compensation is needed for a large distance of the arc-walk which may give a strong influence on the temperature distribution within the fusioning region.